In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at a first node to communicate simultaneously with several other nodes in its coverage area.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and thus communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. Pat. No. 7,072,650 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks,” issued on Jul. 4, 2006, in U.S. Pat. No. 6,807,165 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel,” issued on Oct. 19, 2004, and in U.S. Pat. No. 6,873,839 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System,” the entire content of each being incorporated herein by reference.
As can be appreciated by one skilled in the art, radio frequency (RF) power amplifiers are used in wireless communication networks for the transmission of signals. Solid state RF power amplifiers can be modeled by amplitude-modulation/amplitude-modulation (AM/AM) characteristics, because the amplitude-modulation/phase-modulation (AM/PM) characteristics are negligible. As the input signal amplitude to amplifier is increased, the output signal will begin to saturate at some level in the RF amplifier. The amplitude where the output signal moves from the linear region to the saturation region is usually referred to as the one decibel (1 dB) compression point of the amplifier.
Different models for RF power amplifiers have been developed, and an important feature of each of these models is the manner in which the output signal moves from the linear region to the saturation region. At the extreme end of the model amplifiers are the completely linear model and limiter model. To maintain linear operation, power amplifiers are usually backed off by some number of dBs from the one decibel (1 dB) compression point. The required back-off depends on the crest factor (CF) of the input signal. For orthogonal frequency division multiplexing (OFDM) modulation, back-offs higher than 5 dB are generally used. Typical back-off numbers are in the region of 9-12 dB for 64 OFDM.
As understood in the art, the crest factor (CF) of a signal can be defined as the peak to average amplitude ratio. The peak to average power ratio (PAPR) can be computed from the CF. High CF multi-carrier modulation, such as OFDM, can pose problems for RF power amplifiers because they require high linearity. To maintain linearity, power amplifiers are usually backed-off from their 1 dB compression point so that they can reproduce the high peak powers of the signal without distortion.
The high CF of OFDM results from the individual carrier components being added together at different phases in the Inverse Fast Fourier Transform (IFFT). The CF is relatively independent of the modulation method of the individual carriers when there are many sub carriers. The CF is also relatively independent of the number of sub carriers for practical 32-256 sub carrier OFDM modulations.
If the CF of the signal can be reduced prior to the RF power amplifier, the back-off can be reduced and mean output power can be increased. Numerous CF reduction methods have been developed. Generally, these include selective mapping where multiple information equivalent signals are created and the lowest CF signal is then selected for transmission. Partial transmit signal is a similar approach where multiple partial signals are generated and the most beneficial linear combination is transmitted. In addition, hard and soft clipping methods limit CF by removing or reducing peaks.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.